In the stacking of sheets, generally in conjunction with a transverse cutting of the sheets from a continuous web of the paper or cardboard as the sheets are advanced to a stacking location, it is known to provide indicia between reams or collections of predetermined numbers of sheets in the stack, in the form of tabs or marking strips which are laid into the stack as the stack is formed.
Since the insertion of a marking strip can leave a gap in the region of the edges of the sheets, it is preferred in prior art systems to provide the marker insertion device at the sheet-supply side of the stack, corresponding to the rear of the stack.
In this case, the gap formed as the sheets fall on the stack is used to provide the space into which the marker tab or strip is inserted. An apparatus of this type is described in German Patent 38 23 806.
In stacking devices in which a continuous deposition of sheets if provided even during a stack replacement, i.e. removal of a completed stack, the feed side is generally occupied by stack forming and stack replacement elements to the extent that the arrangement of one or more marker inserters at this side is not possible. This is especially the case when the marker insertion device must be adjusted transverse to the feed direction to accommodate different sizes of the sheets.
German Patent 35 39 099 describes an apparatus in which the marker strips are inserted in a direction opposite to the sheet transport direction for stacking on the freely accessible front side of the stack. A tongue in the form of flat wedge with downwardly inclined flanks can be introduced into the outline of the stack, lowered onto the stack and retracted with only slight lifting out of the outline of the stack before a new introduction of the tongue into position to be lowered onto the stack.
The marker strip is fed from a supply roll through a passage within the tongue opening onto the stack and is cut from the continuous web. The marker strip is thus introduced into a gap in the stack maintained by a downwardly inclined flank of the tongue. As a consequence, as the tongue is retracted below the sheets piled on top of it, it will not pull out the marker strip of paper which has been introduced, especially since the tongue is slightly lifted during the retraction.
Since only the tongue is moved while the guide passage for the marker strip is of fixed height, the strips can only be inserted at a single level of the apparatus. The apparatus cannot accommodate significant changes in height of the stack surface at which the marker strip is to be inserted, for example resulting from bulging at the edges of the sheets because of the trapping of air in the stack or profile fluctuation of the stack or upon stacking on auxiliary stacking platforms during a stack change. As a result, the marker strip insertion can interfere with stacking, the insertion tongue can damage the stack or the stack can cause the tongue to tilt.
In short, while the aforedescribed apparatus can operate relatively efficiently if the marker strip insertion is always to be effected at a single level and the disturbances described do not occur, in the presence of such disturbance factors, it may become unreliable and a detriment to the stacking operation.